With increasing awareness of global environment issues, a thermoplastic resin is easily recycled as compared with a thermosetting resin and the application is expanded year by year.
On the other hand, since the depletion of fossil materials and oil resources and the increase in carbon dioxide have posed a problem, the research and development of a biodegradable resin such as an aliphatic polyester and the like and a resin synthesized by using plants as raw materials have been actively conducted. Among the aliphatic polyesters, a polylactic acid having excellent moldability has especially attracted attention as a resin derived from plant which is produced from lactic acid obtained by fermentation of grain resources such as corns and the like.
A thermoplastic resin generally has an advantage in that it is easily molded by heat and pressure, while it has drawback in heat resistance because the softening occurs at a temperature of the glass transition point (Tg) or higher. As a means to solve this problem, the reinforcement is made by a sub-material such as glass fiber, carbon fiber, talc, silica and the like. However, the heat resistance may be significantly increased by accelerating the crystallization in a resin having crystallinity.
However, since polylactic acid has a slow rate of crystallization and low heat resistance, the use expansion is limited. Particularly, in the case of a polylactic acid amorphous molding article, because it has a softening point of less than 60° C., a problem easily causing whitening, deformation and the like in daily use environments was pointed out.
In addition, if attempts are made to improve the heat resistance of polylactic acid by increasing the crystallinity by heat-treating (annealing), crystals (for example, spherocrystals) having sizes almost equal to or larger than the wavelength of light, which cause scattering of light, generally grow rapidly, and the molded article becomes opaque.
In order to solve these problems, a lot of attempts have been made to improve heat resistance and transparency by adding various additives to polylactic acid.
In patent document 1, it is described that the addition of a phosphate ester metal salt, hydrous magnesium silicate and the like as a nucleating agent is effective. However, when such a nucleating agent is used, polylactic acid has a drawback in that the transparency is impaired. In addition, talc commonly used is practically usable only from the viewpoint of the rate of crystallization. However, to achieve this, the addition amount of talc is often required to be 1% or more, and talc has a drawback of impairing transparency which is an inherent characteristic of polylactic acid.
In patent document 2, there is described a process of adding as a nucleating agent at least one kind of transparent nucleating agent selected from an aliphatic carboxylic acid amide, an aliphatic carboxylic acid salt, an aliphatic alcohol and an aliphatic carboxylic acid ester. However, in this case, the molded article had a crystallinity of 33% and a haze of 6.5%, and the result having both sufficient crystallinity and transparency was not obtained.
In patent document 3, there is described a process of using polylactic acid and an inorganic filler in which a compound having a specific functional group is used as an initiator. However, although the slip properties were improved by this process, the transparency was not maintained because an inorganic filler was added.
Patent document 1: Japanese Patent Laid-Open Publication No. 192884/2003
Patent document 2: Japanese Patent Laid-Open Publication No. 278991/1997
Patent document 3: Japanese Patent Laid-Open Publication No. 285121/2004